Relevant prior art is disclosed in German Offenlegungsschriften 3,902,095; 3,437,253; 2,556,340.
Non-destructive methods for layer thickness measurement, which use the widespread magneto-inductive method or the eddy-current method, are based on the variation in a low-frequency or high-frequency electromagnetic field as a function of a layer applied to the measured object. The field used for measurement has a spatial extent, and thus there is not only a desired dependence on the layer thickness, but also on the shape of the measured object.
In the case of the magneto-inductive, low-frequency method, which is used to measure non-magnetic or electrically non-conductive layers on a magnetic base material, the permeability of the measured object also features in the measurement as a further disturbance variable. In the eddy-current method, which is principally used to measure electrically non-conductive or weakly conductive layers on nonferrous metals, the influence of the geometrical shape of the measured object is substantially more strongly pronounced. Instead of the permeability, which with nonferrous metals can be set virtually equal to 1, the electrical conductivity of the base material also features as a further disturbance variable. It has recently become possible to use circuit engineering in order to exclude this undesired influence over a wide range for the latter disturbance variable.
In both methods, the geometrical shape of the measured object remains a non-negligible influencing variable. It is therefore necessary in measurement practice to perform a so-called calibration on the measured object. This is carried out by simulating the measured value 0 firstly on the non-coated measured object and then on a measured object with a known layer, which is applied either permanently or in the form of a foil on the non-coated measured object. The indicated value corresponding to the measured value is set to the known layer thickness of the measured object. This calibration can be carried out using a plurality of layers in order to match the characteristic better to the relevant measurement task. Since the measured object is not generally available in a non-coated form, and often has very complex shapes, it is mostly very difficult to measure layer thicknesses on concave or convex surfaces. One of the possibilities of reducing the geometrical influencing variables resides in the design of the measuring probe. The smaller the measuring probe, the smaller is the spatial extent of the measuring field and thus the dependence on the geometrical shape of the measured object. However, such measures are limited, since for the layer thicknesses in the range 0-300 .mu.m, which are principally of interest in practice, the probes would have to be kept so small in size that an implementation of such designs is no longer technically possible.